Automatic leveling system for blast hole drills and the like

ABSTRACT

An automatic system for leveling a blast hole drill during raising and lowering includes a front jack on the longitudinal axis of the machine and a pair of rear jacks which straddle the longitudinal axis and define a transverse axis. An operating fluid line for the jacks is connected to a first proportioning valve which is controlled by a first level sensor operative along the longitudinal axis. The proportioning valve apportions flow between the front jack and the two rear jacks and alters the flow ratio as necessary to keep the machine level on the longitudinal axis. A second proportioning valve controlled by a second level sensor operative along the transverse axis is interposed in that output of the first valve which leads toward the two rear jacks. It apportions the flow between the two rear jacks to level the machine along the transverse axis. This sequential apportioning of fluid flow is used to level the machine and keep it level as it is raised and lowered to and from a working position in which it is supported by the jacks.

United States Patent [72] Inventor Thomas A. Stoner Brookiield, Wis.[21] Appl. No. 13,689 [22] Filed Feb. 24, 1970 [45] Patented Dec. 7,1971 [73] Assignee Bueyrus-Erie Company South Milwaukee, Wis.

[54] AUTOMATIC LEVELING SYSTEM FOR BLAST HOLE DRILLS AND THE LIKE 5Claims, 6 Drawing Figs.

[52] US. Cl 254/86 ll, 280/6 H [5 1] Int. Cl 1360s 9/02 [50] Field ofSearch 254/86 R,

[56] References Cited UNITED STATES PATENTS 2,713,402 7/1955 Balogh254/86 H X 3,265,357 8/1966 Schilling... 254/89 1-1 3,431,727 3/1969Grumman 254/86 H X 3,516,182 6/1970 Wykert 280/61 X PrimaryExaminer-Robert C. Riordon Assistant E.raminerDavid R. MeltonAttorneys-Allan W. Leiser, Arthur H. Seidel and Ray G.

Olander ABSTRACT: An automatic system for leveling a blast hole drillduring raising and lowering includes a front jack on the longitudinalaxis of the machine and a pair of rear jacks which straddle thelongitudinal axis and define a transverse axis. An operating fluid linefor the jacks is connected to a first proportioning valve which iscontrolled by a first level sensor operative along the longitudinalaxis. The proportioning valve apportions flow between the front jack andthe two rear jacks and alters the flow ratio as necessary to keep themachine level on the longitudinal axis. A second proportioning valvecontrolled by a second level sensor operative along the transverse axisis interposed in that output of the first valve which leads toward thetwo rear jacks. It apportions the flow between the two rear jacks tolevel the machine along the transverse axis. This sequentialapportioning of fluid flow is used to level the machine and keep itlevel as it is raised and lowered to and from a working position inwhich it is supported by the jacks.

PATENTEDBEU 7197: 3.625483 SHEET 1 UF 3 INVENTOR THOMAS A. STONERATTORNEY PATENTEI) DEC 7191:

SHEET 2 [IF 3 ATTORNEY PATENTED 0E0 H971 3625483 SHEET 3 [IF 3 T 5 51/ Il 57 {5 I I I M 1.. g 27 a I I I INVENTOR THOMAS A. STONER ATTORNEYAUTOMATIC LEVELING SYSTEM FOR BLAST HOLE DRILLS AND THE LIKE BACKGROUNDOF THE INVENTION Blast hole drills are mobile machines used primarily insur face mining to drill holes for explosives. They are generallymounted on crawlers which provide mobility over rough terrain, but it isconventional to provide hydraulic jacks to raise and support the machineoff the crawler mount during the actual drilling operation. It isimperative that the machine be level for drilling to insure that thedrilled hole will be vertical and to prevent unnecessary strain and wearon the machinery. It is also important that the machine be capable ofbeing raised, leveled and lowered quickly to prevent unnecessary delaysbetween drilling operations. It is also highly desirable to have themachine maintained level as it is lowered back to the ground.

Various raising and leveling systems for blast hole drills have beenprovided in the past, but none of these has proven entirelysatisfactory. In many cases, the entire leveling operation must becontrolled manually. These systems require that the operator constantlyobserve and control the attitude of the machine on at least two axes,which is both a difficult and a time consuming task. Automatic levelingsystems have been provided on machines, but these generally involveactuating or controlling one jack or axis at a time resulting insubstantial delay and in an undesirable stepping type of operation.Furthermore, previously devised systems, both manual and automatic, areundesirably complicated and expensive for application to blast holedrills.

SUMMARY OF THE INVENTION The present invention contemplates an automaticleveling system based on the use of proportioning valves whichsequentially apportion fluid flow between jacks located along at leasttwo axes so that the machine is automatically leveled as it is raised.The invention also contemplates reverse use of the proportioning valvesto provide that the machine is leveled as it is lowered back to theground. Further, the invention contemplates a controlled operatingsequence in which pressure-actuated switches are used to determine whena desired percentage of the weight of the machine is being supported bythe jacks and which then initiate a timed final phase of raising, thereare cutoff networks to allow lowering while the machine is on uneventerrain, and there is a pressure device to shut off the lowering cycleautomatically. The circuitry also includes check valve systems toprevent fluid flow from the jacks during drilling. The system disclosedby the invention is highly versatile and readily adaptable to machinesof various sizes, while remaining relatively simple, inexpensive andeasy to operate.

The invention is particularly intended and suitable for blast holedrills. It will be apparent from the description, however, that it isreadily adaptable to various other machines and applications in which itis desired to raise and level a platform or the like.

Other features, objects and advantages of the invention will becomeapparent from the description to follow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic partial sideview of a blast hole drill incorporating the automatic leveling systemof this invention,

FIG. 2 is a schematic view in perspective illustrating the frame or baseplane of the machine of FIG. 1, the supporting jacks, and the referenceaxes to be referred to in the specification,

FIG. 3 is a schematic electrical and hydraulic circuit dia gramillustrating the leveling system in the machine of FIG. 1,

FIG. 4 is a schematic electrical circuit diagram illustrating one cutoutcircuit for the leveling system of FIG. 3,

FIG. 5 is a schematic electrical circuit diagram illustrating anothercutout circuit for the leveling system of FIG. 3, and

FIG. 6 is a schematic hydraulic circuit diagram illustrating a checkvalve system for the jacks of the leveling system of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT The blast hole drill shown inFIG. I is designated generally by the reference numeral 1, and becauseit is of a generally conventional construction well known to thoseskilled in the art it is not shown and will not be described in detail.The machine proper is supported on a crawler vehicle 1' by means ofwhich it may be moved from location to location. It is supported inworking position and raised and lowered to this position by means ofthree conventional double acting hydraulic jacks, comprising a frontjack 2, a left rear jack 3, and a right rear jack 4 which is not visiblein FIG. I. The jacks 2, 3 and 4 are mounted on a main frame 5 which issupported on the crawler 1 and which serves as a platform for the drillmast 6 and operating machinery 7. The frame 5 also serves to define abase plane for the machine I which must be leveled in order for themachine as a whole to be level.

As can be seen most clearly in FIG. 2, the front jack 2 is at theforward end of a central, longitudinal axis 8. The rear jacks 3 and 4are at the rear end of the axis 8 and straddle it to define a transverseaxis 9. Both axes are in the frame 5 and define the base plane. Thearrow 10 represents a vertical gravitational force vector. The plane 5defined by axes 8 and 9 is shown in FIGS. 1 and 2 ma level workingposition, perpendicular to the gravitational force vector 10. The jacks2, 3 and 4 together form a tripod support for the frame 5, and when theyare suitably extended they serve to lift the frame 5, crawler 1, mast 6and machinery 7 off of the ground to working position.

FIG. 3 shows the basic electrical and hydraulic circuitry used tooperate the jacks 2-4 to raise and lower the machine 1 to and from itsworking position. The reference numeral 11 designates a reservoir ofhydraulic fluid, and the reference numeral 12 indicates a conventionalclosed center directional control valve which serves as a main operatingvalve. A pump 13 serves to move fluid under pressure from the reservoir11 to the valve 12 through a line 13'. An exhaust line 14 allows fluidto move from the valve 12 back to the reservoir 11. A branched returnline 15 leads from the rod ends of the jacks 2-4 to one port of thevalve 12. A main operating line 16 leads from the other port of thevalve I2 and conducts fluid under pressure from the valve 12 during araising operation.

The operating line 16 leads to a first proportioning valve 17 havingfirst and second output lines 18 and 19. A proportioning valve is onewhich apportions fluid flow between two or more outlets and which can beoperated to vary the flow ratio, and various types of proportioningvalves are known to those skilled in the art. In the preferredembodiment of this invention the proportioning valves comprise generallyconventional open center, self-centering directional control valves withtwo motor ports, but one of the two exhaust ports is capped and theother exhaust port is connected to an operating line, the linel6 in thecase of the valve 17. The valve is set so that there is a selectednormal position with a normal flow ratio, and movement of the spool ineither direction will reapportion the flow or in other words vary theflow ratio in favor of one outlet or the other. This action iscontinuous in nature, and is also reversible in that the flow ratio ineither direction through the valve can be controlled. It should beappreciated that the normal position of the proportioning valve 17 maynot be exactly its center position since it may be necessary to accept aslight constant error, as is common in servosystems. Normal" positionsas used herein is intended to mean the position the valve is in when itscontrol axis is level. An electrical operating coil 20 is provided toeffect such operation of the valve 17.

The first outlet line 18 leads directly to the cylinder end of the frontjack 2, and fluid passing therethrough causes the jack 2 to be extendedto raise the machine 1. The second outlet line 19 leads toward the rearjacks 3 and 4, but is connected to a second proportioning valve 24. Thevalve 24 is substantially identical to the proportioning valve 17, andit has a right outlet line 25 and a left outlet line 26 which lead tothe cylinder ends of the right rear jack 4 and the left rear jack 3,respectively. Fluid under pressure passing through these outlet linesserves to extend the jacks 3 and 4 thus raising the machine 1. The valve24 is controlled in its operation by a coil 29 which imparts reciprocalmovement to the valve spool.

A first level sensor unit designated generally by the reference numeral30 and a second level sensor unit designated generally by the referencenumeral 31 are provided to control, respectively, the firstproportioning valve 17 and the second proportioning valve 24. Thiscontrol is exercised by passing current through the coils 20 and 29.Except for their orientation on the frame the two level sensor units aresubstantially identical so that like reference numerals have been usedto refer to like parts. Each unit includes a pendulum 32 which ismounted to swing in a single vertical plane and move a contact 33 alonga resistance element 34 as it swings. The resistance element 34 is partof a signal circuit which generates an error signal indicating thedirection and amount of movement of the contact 33 away from the centerof the resistance element 34. This output is fed to a conventionalamplifier 35, and the outputs of the two amplifiers 35 are connected byoperating circuitry to the coils 20, 29 as will be described. This typeof sensor is quite satisfactory but other sensors, such as resolvers,could be substituted.

Both level sensors are mounted on the base plane 5. The pendulum 32 ofthe first level sensor unit 30 is arranged to swing in the verticalplane defined by the longitudinal axis 8, so that its position indicatesthe attitude of the base plane 5 along this axis. The unit 30 isconnected to the coil through normally closed switches 40 and 41 tocontrol the first proportioning valve 17. The pendulum 32 of the secondsensor unit 31 is arranged to swing in the vertical plane defined by thetransverse axis 9 to indicate the attitude of the plane 5 along thisaxis. The output of the unit 31 is connected to the coil 29 throughnormally closed switches 43 and 44 to operate and control the secondproportioning valve 24.

The two units 30, 31 continuously sense the attitude of the frame orbase plane 5 and therefore the attitude of the entire machine 1. Theoutputs of the amplifiers 35 indicate the amount and direction that thebase plane 5 is tilted from a level attitude. By actuating the coils 20and 29, these outputs continuously control the first and secondproportioning valves 17, 24 which in turn control the flow ratio offluid to the jacks 2-4 in a manner that will be described below.

To prevent the starting or continuation of a raising cycle when themachine is at too great an angle, excess error relays (not shown) can beconnected across the amplifiers 35 or, preferably, across additionalamplifiers (not shown) which are designed for excess error operation.These can be used to break the main operating circuit and initiate alowering cycle, as will be apparent to those skilled in the art.

A main operating coil 45 is provided for the main operating valve 12,and is connected to a source of power 46 through a manually operableswitch 47. A second or reverse operating coil 48 is also provided forthe valve 12, and is connected to power source 46 through a secondmanually operable switch 49. Both switches 47 and 49 are preferably held"on" by a suitable locking circuit (not shown) after being closed. Thereverse operating coil 48 is connected in series with a reversing relaycoil 50. When reversing relay coil 50 is actuated, it opens the switches40, 41, 43 and 44, and simultaneously closes normally open switches 52,53, 54 and 55. This effectively reverses the connections of the levelsensor units 30 and 31 with the respective coils 20 and 29 on theproportioning valves 17 and 24.

Pressure-sensing devices 56, 57 and 58 are connected, respectively, tothe first output 18 of the valve 17 and the first and second outputs 2Sand 26 of the valve 24. These are conventional types of devices in whichthere is a diaphragm exposed to pressure in the line with a microswitchwhich is closed when a given pressure is reached. The pressure sensors56-58 are connected with their switches in series between a DC voltagesource and a time delay relay 60, this arrangement providing a shutoffmeans for terminating a raising cycle. When all three switches close thetime delay relay coil 59 is energized which, in turn, actuates anormally closed switch 61 in series with the manual switch 47. Thesensors 56-58 are preset to close their switches when their respectivejacks are supporting approximately percent of the weight they areintended to support in the fully raised position. When all three sensorsread this pressure the time delay shutoff coil 59 actuates the switch 61after a designated time delay and thus terminates the raising operationby deactivating the main operating coil 45. Another pressure operatedswitch 62 is connected in series with the reverse coil 48 and serves tostop the lowering action upon a predetermined pressure in the line 15which indicates that all of the jacks have been fully raised.

Cutout circuits 63 and 64, the locations of which are shown in brokenlines in FIG. 3 and which are themselves shown in FIGS. 4 and 5, areinterposed in the operating circuitry for the valves 17, 24. These servein effect to disconnect the level sensors 30, 31 from the valves 17, 24during lowering when certain conditions occur, allowing the valves 17,24 to return to centered position, which they will do automatically whenthey are deenergized. The purpose of the cutout circuits 63, 64 is toterminate leveling action when the machine is on uneven terrain and oneportion touches down before the rest. Absent any such control, if, forexample, the front of the machine were to touch down first, furtherlowering would cause the machine to tip down toward the rear, and thiswould cause an error signal which would cause the valve 17 to shift soas to restrict flow from the rear jacks 3 and 4, which are the jacksfrom which there must be flow to lower the machine.

To avoid this problem, the cutout circuits 63 and 64 terminate levelingaction along either axis when one end of that axis in on the ground.This means that the associated valve 17 or 24 will center, which willallow flow through both sides so that lowering can continue insatisfactory fashion. In the preferred embodiment, this is accomplishedthrough normally closed switches in the operating circuitry which areopened in response to low pressure detected by the pressure devices56-58; it is known to those skilled in the art that these devices can bemade to read both high and low pressure and that they can activate anumber of switches. In FIG. 4, the reference numerals 56-58 have beenused for the pressure devices proper, and corresponding prime and doubleprime numbers are used to indicate respective additional switchesoperated by each.

In cutout circuit 63, seen in FIG. 4, switches 57' and 58, openable bydevices 57 and 58, respectively, are connected in series with each otherand the coil 29, which controls the valve 24. The switches 57 and 58 areboth shunted by a switch 47 which closes automatically upon closing ofthe switch 47, as the result of which the cutout circuit 63 isdeactivated during a raising action. The switches 57' and 58' are set toopen when the associated device reads a predetermined low or zeropressure, which indicates that the associated jack 3 or 4 is no longersupporting any weight, or in other words that the left or right side ofthe machine has touched down. Opening of either switch 57' or 58' breaksthe circuit for the coil 29, whereupon the valve 24 will center andallow continued normal flow.

The circuit 64, seen in HQ 5, controls the valve 17, and includes aparallel combination of normally closed switches 57" and 58" (operatedby the devices 57 and 58, respectively) in series with the coil 20 and anormally closed switch 56" (operated by the device 56). A switch 47",also closed automatically when the switch 47 closes, shunts all of theswitches and deactivates the circuit 64 during raising. All of theswitches 56-58" will be opened when the associated device 56-58 readslow or zero pressure. If the rear end of the machine touches down first,switches 57" and 58" will open, and if the front touches down switch 56"will open; in either event the coil 20 will be deactivated and the valve17 will center. The reason for putting switches 57" and 58" in parallelis of course that leveling should not be stopped if only one rear comerhas touched down.

During drilling it is desirable to close off the lines leading to therod and cylinder ends of the jacks 2, 3 and 4 to prevent fluid flow.FIG. 5 illustrates how this is accomplished for the cylinder 4, andsimilar systems are used for the other cylinders. As shown in FIG. 5,normally closed pilot operated check valves 65 and 66 are connected inthe fluid lines leading to the cylinder and rod ends of jack 4, with thepilot line of each check valve 65 and 66 connected to the others fluidline. With this arrangement, the check valves 65 and 66 do not interruptfluid flow when fluid flows toward the jack 4 during raising orlowering, but they prevent fluid from leaving jack 4 when the pump 13 isshut off.

OPERATION When it is desired to raise the machine to the position ofFIG. ll, the switch 47 is closed, activating the coil 45 and causing thespool in the main operating valve 12 to shift to a position in whichfluid under pressure moves through the output line 16 to the firstproportioning valve 17 and returns from line through valve 17 andexhaust line 14 to the reservoir 11. Assuming that the valve 17 is inits normal position the fluid flow will be distributed between theoutput lines 18 and 19 in a ratio which will cause the jacks 2, 3 and 4to be extended at the same rate.

The actual flow rate will depend upon the relative piston areas of theseveral jacks. Assuming jacks of equal size this means that in thenormal position one-third of the flow would be sent through the outletline 18 and two-thirds would be sent through the outlet line 19 becausethe latter supplies fluid to two jacks 3 and 4. If, however, the machineis not level along the longitudinal axis 8 the pendulum 32 in the. levelsensor 30 will shift, generating a signal to the coil 20 which willproportionally shift the spool in the valve 17 to alter the flow rate tocompensate for the nonlevel attitude of the machine. The sensing actionof the unit 30, its control of the coil 20, and the actuation of thevalve 17 is a continuing operation so that the machine is promptlybrought to and maintained in a level attitude along the axis 8throughout the raising operation.

The fluid flow through the outlet line 19 is fed into the secondproportioning valve 24 and, assuming the jacks 3 and 4 are of the samesize, it is fed equally through the output lines 25 and 26 when thevalve 24 is in its nonnal position. Deviation from level about thetransverse axis 9 will cause movement of the pendulum 32 in the levelsensor 31 which results in a signal to the coil 29 proportionallyshifting the spool in the valve 24 to alter the flow rate. Thisalteration compensates for the deviation and brings the machine back tolevel along the transverse axis 9. Again, the sensing, control andactuation is a continuous operation so that the machine is brought toand maintained in the level attitude along the transverse axis 9throughout the raising operation.

When the jacks are supporting 80 percent of the machine weight, thepressure sensors 56-58 close their switches and energize the coil 59which will, after a preset time delay, open the switch 61 to terminatethe raising action. The time delay is preset to allow the jacks toextend to the point where they support 100 percent of the machineweight.

The operation during lowering is essentially the same, except the flowof fluid is reversed. To begin lowering, the switch 49 is closed causingthe valve 12 to be actuated in the opposite direction. Fluid underpressure is then sent through the line 15 to the rod ends of the jacks2-4 and fluid from the cylinder ends of said jacks will returnultimately through the line 16. Fluid flows from the jacks 3 and 4through the lines 25 and 26 and through the second proportioning valve24 at an equal rate when the valve 24 is in its normal position. If,however, the machine deviates from a level position along the axis 9 thelevel sensor unit 31 will generate a signal causing the valve 24 toalter the equal flow rate. The connections for the unit 31 to the coil29 have been reversed by the reversing relay 51 so that the signalgenerated by the level sensors will result in a shifting of the spoolsin the proportioning valves in the opposite direction from that whentilting occurs during the raising operation. Return flow through thelines 18 and 19 and through the valve 17 will also be in the normalratio when the valve 17 is in its normal position. If there is adeviation along the longitudinal axis 8 the valve 17 will be actuated toalter the flow rate and maintain the machine level.

As discussed above, the circuits 63 and 64 cut off the leveling actionwhen one portion of the machine is completely lowered to the groundbefore the others. Once the crawler 1 is resting entirely on the groundthere is a pressure flow through the line 15 for a short period of timeso that the jacks 2-4 will reach the upper limit of their travel, andthis is shut off by pressure-operated switch 62.

The system of the invention is easy to operate and relativelyinexpensive. It also provides highly accurate, continuous and extremelyquick-acting control of machine attitude. A preferred embodiment of theinvention has been shown and described herein, but it will be obviousthat various modifications might be made without departure from thespirit of the invention. Various types of level sensors and pressuresensors, all of which are well known to those skilled in the art, mightbe substituted for the particular forms shown. Modifications might alsobe made in the electrical circuitry, elements may be added or combined,and different types of proportioning valves might be used. Further, theinvention is not necessarily restricted to the use of three jacks on twoaxes. It is possible, for example, to have four jacks on three axes. Toaccomplish this, it would be necessary only to replace the front jack 2with a pair of jacks situated like the jacks 3 and 4 and to interpose athird proportioning valve like the valve 24. The result would be thatthe structure at the end of the output line 18 would be essentially amirror image of that now shown in FIG. 3 at the end of the line 19. Thethird proportioning valve could be controlled by the same transverselevel sensor unit 31, or by its own sensor. As a further alternative,two slightly transversely spaced jacks both fed ofi the same output linecould be substituted for the single front jack 2, although these wouldoperate essentially as a single jack. Still further, it is not necessarythat the actual central longitudinal axis of the machine be used as areference or that a perpendicular transverse axis be selected as areference for leveling. In essence, it is necessary only that there betwo jack means each aligned to define one axis in the base plane of themachine. One of the two jack means should be comprised of at least twojacks aligned to define a second axis in the base plane which intersectsand is at least generally transverse to the first axis. In view of theseveral possible modifications, it is not intended that the invention belimited by the showing herein or in any other manner except as mayspecifically be required.

I claim:

1. In a leveling system for a blast hole drill or the like having a baseplane, the combination comprising:

a first and a second jack means which are spaced from one another andtogether define a first axis in the base plane, the second jack meanscomprising a pair of spaced jacks defining a second axis in the baseplane that is generally transverse to the first axis;

a fluid supply line;

a first proportioning valve in the supply line which has a first outputleading to the first jack means and a second output leading toward thesecond jack means, said first proportioning valve being operable tocontinuously apportion the flow of fluid to the first and second jackmeans to maintain the base plane level along said first axis;

a first level sensor means adapted to continuously detect the attitudeof the base plane along the first axis and operate the firstproportioning valve in proportional response thereto;

a second proportioning valve connected to the second output of the firstproportioning valve, said second proportioning valve having two outputsleading to respective members of the second jack means and beingoperable to continuously apportion the flow of fluid to the two jacks tomaintain the base plane level along the second axis; and

a second level sensor means adapted to continuously detect the attitudeof the base plane along the second axis and operate the secondproportioning valve in proportional response thereto.

2. The combination of claim 1 wherein both proportioning valves havenormal positions which are assumed when the base plane is level and saidproportioning valves can be actuated in either direction from saidnormal position; and the ratio of flow through the first and secondoutputs of the first proportioning valve when it is in normal positionis approximately equal to the ratio of the piston area of the first jackmeans to the total piston area of the jacks of the second jack means;and the ratio of flow through said outputs of the second proportioningvalve when it is in normal position is approximately equal to the ratioof the respective piston areas of the jacks of the second jack means.

3. The combination of claim 2 wherein the proportioning valves areadapted to control the flow ratio of fluid moving through them in eitherdirection; and the sensors are adapted to generate signals correspondingin polarity and magnitude to the attitude of the base plane along theirrespective axes; and there is a first set of connections between eachsensor and the associated proportioning valve which is used during araising action; and there is a second set of connections between eachsensor and the associated proportioning valve which is used during alowering action and in which the polarity of the signal to the valve isreversed; and there is switch means to select which set of connectionsis to be used.

4. The combination of claim 3 wherein there are three pressure-sensingdevices, one associated with the first output of the first proportioningvalve and the other two associated, respectively, with the first andsecond outputs of the second proportioning valve, each device beingadapted to be actuated upon the occurrence of a predetermined lowpressure in its associated output which indicates that the associatedjack or jack means is no longer supporting a substantial load during alowering cycle; and there is an operating circuit between each levelsensor unit and its associated proportioning valve; and eachproportioning valve has a self-centering action so that upon aninterruption in its operating circuit it will return to centeredposition; and there is a first cutoff circuit in the operating circuitfor the first proportioning valve which comprises a parallel combinationof two normally closed switches, one adapted to be opened upon actuationof one of said other two devices and the other adapted to be opened uponactuation of the other of said other two devices, said combination beingconnected in series with a third normally closed switch adapted to beopened upon actuation of said one device; and there is a second cutoflcircuit in the operating circuit for the second proportioning valvewhich comprises a normally closed switch adapted to be opened uponactuation of one of said other two devices and connected in series withanother normally closed switch which is adapted to be opened uponactuation of the other of said other two devices; and each cutoffcircuit includes means to deactivate the same during a raising cycle.

5. The combination of claim 4 wherein each of the three pressure-sensingdevices is also adapted to be actuated when a preselected percentage ofthe total weight of the drill is supported by its associated jack orjack means; and there is a shutoff circuit which is adapted to terminatefluid flow through said fluid supply line, the shutoff circuit includingthree normally open switches connected in series, each switch beingadapted to be closed upon actuation of a respective device, and a timedelay means which, upon closing of all three switches causes flow to beterminated after a predetermined time interval.

1. In a leveling system for a blast hole drill or the like having a baseplane, the combination comprising: a first and a second jack means whichare spaced from one another and together define a first axis in the baseplane, the second jack means comprising a pair of spaced jacks defininga second axis in the base plane that is generally transverse to thefirst axis; a fluid supply line; a first proportioning valve in thesupply line which has a first output leading to the first jack means anda second output leading toward the second jack means, said firstproportioning valve being operable to continuously apportion the flow offluid to the first and second jack means to maintain the base planelevel along said first axis; a first level sensor means adapted tocontinuously detect the attitude of the base plane along the first axisand operate the first proportioning valve in proportional responsethereto; a second proportioning valve connected to the second output ofthe first proportioning valve, said second proportioning valve havingtwo outputs leading to respective members of the second jack means andbeing operable to continuously apportion the flow of fluid to the twojacks to maintain the base plane level along the second axis; and asecond level sensor means adapted to continuously detect the attitude ofthe base plane along the second axis and operate the secondproportioning valve in proportional response thereto.
 2. The combinationof claim 1 wherein both proportioning valves have normal positions whichare assumed when the base plane is level and said proportioning valvescan be actuated in either direction from said normal position; and theratio of flow through the first and second outputs of the firstproportioning valve when it is in normal position is approximateLy equalto the ratio of the piston area of the first jack means to the totalpiston area of the jacks of the second jack means; and the ratio of flowthrough said outputs of the second proportioning valve when it is innormal position is approximately equal to the ratio of the respectivepiston areas of the jacks of the second jack means.
 3. The combinationof claim 2 wherein the proportioning valves are adapted to control theflow ratio of fluid moving through them in either direction; and thesensors are adapted to generate signals corresponding in polarity andmagnitude to the attitude of the base plane along their respective axes;and there is a first set of connections between each sensor and theassociated proportioning valve which is used during a raising action;and there is a second set of connections between each sensor and theassociated proportioning valve which is used during a lowering actionand in which the polarity of the signal to the valve is reversed; andthere is switch means to select which set of connections is to be used.4. The combination of claim 3 wherein there are three pressure-sensingdevices, one associated with the first output of the first proportioningvalve and the other two associated, respectively, with the first andsecond outputs of the second proportioning valve, each device beingadapted to be actuated upon the occurrence of a predetermined lowpressure in its associated output which indicates that the associatedjack or jack means is no longer supporting a substantial load during alowering cycle; and there is an operating circuit between each levelsensor unit and its associated proportioning valve; and eachproportioning valve has a self-centering action so that upon aninterruption in its operating circuit it will return to centeredposition; and there is a first cutoff circuit in the operating circuitfor the first proportioning valve which comprises a parallel combinationof two normally closed switches, one adapted to be opened upon actuationof one of said other two devices and the other adapted to be opened uponactuation of the other of said other two devices, said combination beingconnected in series with a third normally closed switch adapted to beopened upon actuation of said one device; and there is a second cutoffcircuit in the operating circuit for the second proportioning valvewhich comprises a normally closed switch adapted to be opened uponactuation of one of said other two devices and connected in series withanother normally closed switch which is adapted to be opened uponactuation of the other of said other two devices; and each cutoffcircuit includes means to deactivate the same during a raising cycle. 5.The combination of claim 4 wherein each of the three pressure-sensingdevices is also adapted to be actuated when a preselected percentage ofthe total weight of the drill is supported by its associated jack orjack means; and there is a shutoff circuit which is adapted to terminatefluid flow through said fluid supply line, the shutoff circuit includingthree normally open switches connected in series, each switch beingadapted to be closed upon actuation of a respective device, and a timedelay means which, upon closing of all three switches causes flow to beterminated after a predetermined time interval.